1. Field of the Invention
The present invention relates to displacement measurement techniques, instrumentation techniques, evaluation techniques, precision patterning techniques, fine patterning techniques, semiconductor patterning techniques, and master mask patterning techniques. More particularly, the present invention relates to a displacement measurement technique which requires accuracy of the order of nanometer.
2. Description of the Related Art
For example, a laser interferometer displacement measuring system is often used as high accuracy displacement measurement means for controlling such as a stepper, employed in the photolithography process for fabricating semiconductor devices, and for controlling X-Y stages for use such as in precision machining equipment. A nominal value of resolution is 0.3 nm for the displacement measurement system which provides the most accurate displacement measurement and has been developed particularly for stepping control.
Concerning the provision of increased accuracy, even a general displacement measurement technique that does not employ the laser interferometer displacement measuring technique but employs noise processing by averaging is disclosed in Japanese Patent Laid-Open Publication No. Hei 7-306034 in relation to the non-contact displacement measurement. Optical measurement employing an optical interferometer is disclosed in Japanese Patent Laid-Open Publication No. Hei 9-178567 in relation not to position but to wavelength measurement.
However, in many cases, even the current laser interferometer displacement measuring system having a nominal value of resolution of 0.3 nm actually provides only an absolute accuracy of the order of xc2x12 nm for displacement measurement. The resolution and the absolute accuracy are essentially different from each other. An interferometer displacement measuring system may apparently have an accuracy of 0.3 nm in the range of about 10 nm, but in some cases, a gradual undulation may be generally found with the magnitude reaching 3 nm or more in the range of 100 to 300 nm. These problems were not made clear until a high-speed real-time displacement measurement approach was developed to thereby make it possible to measure the displacement of a moving object with high accuracy at a frequency greater than that of mechanical vibrations.
In general, to provide an increased accuracy, the noise processing by averaging over time is performed as mentioned above in order to improve the accuracy (i.e., relative accuracy) of stability of measurement values under a standstill condition of the object. However, with a recent increasing demand for increased accuracy, the absolute accuracy of measurement values has become necessary. In the course of study to the present invention, such a problem has become clear that the prior-art noise processing by averaging cannot provide a sufficient absolute accuracy.
In view of the aforementioned problems, it is therefore the object of the present invention to provide a high-accuracy interferometer displacement measuring system which provides an absolute accuracy in the range of xc2x12 nm to xc2x11 nm or less for a displacement measurement value using interference of laser light.
In consideration of the fact that the interference of light itself, which is the principle of laser interferometry, causes an error, the present invention is adapted to eliminate errors, concerning absolute accuracy, which cannot be eliminated only by averaging over time. Approaches to increased accuracy that focus attention on such a cause of error have never discussed before. More specifically, a correction value corresponding to the laser wave cycle of a displacement is added to a displacement output of the laser interferometer displacement measuring system, thereby correcting the distortion error in the interferometer displacement measuring system.
Upon measurement of a continuously moving object as a measurement target, the laser interferometer displacement measuring system according to the present invention stores and corrects, as a measurement error caused by the interference effect, an oscillatory component that appears in the cycle consistent with the frequency of laser light, thereby implementing an increased accuracy. Even such a correction method for allowing a relatively simple sinusoidal wave to be added to or subtracted from a measurement value can reduce the range of error of absolute position about xc2x12 nm to within xc2x11 nm, thereby making it possible to provide an increased accuracy.
That is, a laser interferometer displacement measuring system according to the present invention is characterized by comprising a displacement measurement mechanism making use of laser interference, and corrector means for adding a correction value to or subtracting the correction value from a measurement value of the displacement measurement mechanism. The corrector means uses a cyclic correction value having a cycle corresponding to a wave cycle of laser light.
Furthermore, a laser interferometer displacement measuring system according to the present invention is characterized by comprising a displacement measuring mechanism making use of laser interference, and corrector means for adding a correction value to or subtracting the correction value from a measurement value of the displacement measurement mechanism. The corrector means has storage means for storing a cyclic correction value having a cycle corresponding to a wave cycle of laser light, and the correction value is read out of the storage means in accordance with the measurement value and is added to or subtracted from the measurement value. It is possible to employ a rewritable memory as the storage means.
Furthermore, a laser interferometer displacement measuring system according to the present invention comprises a laser light source, an interferometer for dividing laser light of wavelength xcex emitted from the laser light source into a reference path beam and a measurement path beam to interfere the reference path beam with the measurement path beam having been reflected from a subject body, a light detector for detecting the light subjected to the interference in the interferometer, and measurement value output means for converting a detection signal of the light detector into a measurement value to output the resulting value. In the system, a displacement of the subject body causes an n-fold variation in length of an optical path between the interferometer and the subject body. The laser interferometer displacement measuring system is characterized by further comprising corrector means for adding a correction value to or subtracting the correction value from the measurement value of the measurement value output means. The system is also characterized in that, with the measurement value being employed as a variable, the corrector means uses, as the correction value, a cyclic function having a cycle of xcex/n or a sum of a plurality of cyclic functions having the cycle of xcex/n as a fundamental cycle. The plurality of cyclic functions having the cycle of xcex/n as a fundamental cycle can be the cyclic function having a cycle of xcex/n and harmonic cyclic functions thereof.
The aforementioned laser interferometer displacement measuring system according to the present invention can comprise means for performing feedback control so as to carry out tracking adjustment of a phase and amplitude of the correction value.
A laser interferometer displacement measuring system according to the present invention is characterized by comprising a displacement measurement mechanism making use of laser interference, and corrector means for adding a correction value to or subtracting the correction value from a measurement value of the displacement measurement mechanism. The corrector means prepares a plurality of types of cyclic functions having a cycle corresponding to a wave cycle of laser light, and assigns weights to each of the cyclic functions to allow the resulting cyclic functions to be added to or subtracted from the measurement value.
It is also possible to employ mathematically orthogonal cyclic functions as the plurality of types of correction value cyclic functions. For example, as the mathematically orthogonal cyclic function, it is possible to use the sinusoidal (sin) and cosine (cos) functions and a group of harmonics of these cyclic functions. In addition, as the plurality of types of correction value cyclic functions, it is possible to use a triangular wave function and a group of orthogonal harmonic cyclic functions of the triangular wave. Incidentally, the plurality of types of correction value cyclic functions do not have necessarily to be mathematically orthogonal cyclic functions.
To use functions that are orthogonal to each other as the plurality of types of cyclic functions, the system can be provided with calculation means for calculating the magnitude of the component of each cyclic function in the cyclic error contained in a measurement value by integrating each cyclic function individually with the measurement value to perform the weighting by means of the output of the calculation means.
Furthermore, the system can be provided with phase shift means for shifting the phase of the plurality of types of cyclic functions at the same time to perform feedback control on the amount of shift provided by the phase shift means.
It is preferable to make the feedback time constant of the amount of shift provided by the phase shift means shorter than the feedback time constant of other amplitudes. It is preferable to provide the system with means for enabling the feedback control only when the subject body is moving at a given speed or greater.
Furthermore, it is possible to dispose averaging means capable of averaging over time after the aforementioned corrector means. It is also possible to configure the system to bypass the averaging processing provided by the averaging unit means for output.
Furthermore, a laser interferometer displacement measuring system according to the present invention is characterized by comprising a displacement measurement mechanism making use of laser interference, and error signal component generating means for eliminating a constant speed component and an acceleration component from a measurement value of the displacement measurement mechanism and generating an error signal component. The system further comprises storage means for storing the error signal component generated from the error signal component generating means corresponding to the measurement value, and means for allowing the error signal component stored in the storage means to be added to or subtracted from the measurement value of the displacement measurement mechanism as a correction value.
Furthermore, a laser interferometer displacement measuring system according to the present invention comprises a laser light source, an interferometer for dividing laser light of wavelength xcex emitted from the laser light source into a reference path beam and a measurement path beam to interfere the reference path beam with the measurement path beam having been reflected from a subject body, a light detector for detecting the light subjected to the interference in the interferometer, and measurement value output means for converting a detection signal of the light detector into a measurement value to output the resulting value. In the system, a displacement of the subject body causes an n-fold variation in length of an optical path between the interferometer and the subject body. The laser interferometer displacement measuring system is characterized by further comprising corrector means for adding a correction value to or subtracting the correction value from the measurement value of the measurement value output means. The corrector means comprises means for storing or calculating, with the measurement value being employed as a variable, a cyclic function having a cycle of xcex/n or the cyclic function having a cycle of xcex/n and harmonic cyclic functions thereof, and error signal component generating means for eliminating a constant speed component and an acceleration component from a measurement value of the displacement measurement mechanism and generating an error signal component. The corrector means further comprises adjustment means for adjusting an amplitude and a phase of the cyclic function so that the cyclic function having a cycle of xcex/n or a sum function of the cyclic function having a cycle of xcex/n and harmonic cyclic functions thereof fits to the error signal component, and means for allowing a function value of the cyclic function having a cycle of xcex/n or a function value of the sum function of the cyclic function having a cycle of xcex/n and harmonic cyclic functions thereof to be added to or subtracted from the measurement value.
The system can be configured such that correction processing is carried out by means of hardware in a real time manner or implemented by software means in conjunction with a mechanism for calculating correction values.
A laser interferometer displacement measuring system according to the present invention makes it possible to drive a displacement measurement subject at a given speed upon activation or initialization of the system to acquire correction data at that time. In addition, a laser interferometer displacement measuring system according to the present invention may be adapted to be combined with a stage control device to set a correction value of a correction table for use in laser displacement measurement at the time of initial operation or control of the stage.
The measurement value correcting means or means for implementing the method for correcting measurement values can be integrated on a measuring board (counter board or axis board) for laser interferometry.
A laser interferometer displacement measuring system according to the present invention can be mounted on a single-axis stage, a multi-axis stage, or an X-Y stage.
An apparatus according to the present invention comprises a stage for placing thereon and moving a sample or a subject work, drive means for driving the stage, and a laser interferometer displacement measuring system for measuring a position of the stage. The apparatus is characterized in that, as the laser interferometer displacement measuring system, the aforementioned laser interferometer displacement measuring system is employed. Examples of those apparatuses include an electron beam lithography system, a stepper for fabricating semiconductors (an exposure apparatus), a fine patterning system, metal machining equipment, ceramic machining equipment, mask pattern transfer equipment, mask patterning equipment, an electron-beam scanning microscope with a displacement measurement function, a transmission electron microscope with a displacement measurement function, and non-contact shape measurement equipment.
A laser interferometer displacement measuring system according to the present invention comprises a light detector for detecting the light subjected to interference, phase detector means for detecting a phase from a detection signal of the light detector, accumulator means for accumulating variations in phase obtained from the phase detector means, correction value generating means for generating a correction value from an accumulated value provided by the accumulator means or the phase value, and corrector means for allowing the correction value, generated by the correction value generating means, to be added to the accumulated value or the phase value. The correction value generating means generates a cyclic correction value of wavelength xcex of laser light with the accumulated value or the phase value being employed as a variable and eliminates a signal component produced in phase with the wave cycle of the laser light.
The correction value generating means generates a correction value with an accumulated value, not a phase value, or variations in phase value being employed as a variable, thereby generating a cyclic correction value at a cycle of xcex independent of n. The correction value is generated in the cycle of xcex, thereby extracting the error of a plurality of cyclic components corresponding to each harmonic component of 1 to 2 n harmonic waves of wavelength xcex.
The laser interferometer displacement measuring system according to the present invention comprises means for suppressing a relative peak intensity, with respect to a baseline of a frequency spectrum, of a peak of frequency component f=Nv/xcex (N is a natural number of 1 to 2 n and not equal to n) of a signal generated in the light detector due to a movement of the subject body at speed v. This allows for eliminating those frequency components to provide increased accuracy for the laser interferometer displacement measuring system.